1. Field of the Invention
The present invention relates to a process for the desulfurization of a light boiling range (C5-350xc2x0 F.) naphthas, such as fluid catalytic cracked naphtha. More particularly the present invention employs a combination of steps which include catalytic distillation to reduce sulfur to very low levels, makes more efficient use of hydrogen and causes less olefin hydrogenation for a full boiling range naphtha stream.
2. Related Information
Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges which determine the composition. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule most of the xe2x80x9csweetxe2x80x9d (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates and cyclic and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha as it comes from the catalytic cracker has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane.
Catalytically cracked naphtha gasoline boiling range material currently forms a significant part (≈⅓) of the gasoline product pool in the United States and it provides the largest portion of the sulfur. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations. Environmental concerns suggests that the sulfur of the final product should be below 50 wppm.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally copious quantities of hydrogen are included in the feed. The following equations illustrate the reactions in a typical naphtha HDS unit:
RSH+H2xe2x86x92RH+H2Sxe2x80x83xe2x80x83(1)
RCI+H2xe2x86x92RH+HCIxe2x80x83xe2x80x83(2)
RN+2H2xe2x86x92RH+NH3xe2x80x83xe2x80x83(3)
ROOH+2H2xe2x86x92RH+2H2Oxe2x80x83xe2x80x83(4)
Typical operating conditions for the naphtha HDS reactions are:
After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the now desulfurized naphtha. The loss of olefins by incidental hydrogenation is detrimental by the reduction of the octane rating of the naphtha and the reduction in the pool of olefins for other uses.
In addition to supplying high octane blending components, the cracked naphthas are often used as sources of olefins in other processes such as etherifications. The conditions of hydrotreating of the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction reducing the octane and causing a loss of source olefins.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Since the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction, the most common solution has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C5 to about 250xc2x0 F. and a heavy boiling range naphtha which boils in the range of from about 250-475xc2x0 F.
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. However, in the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans, the mercaptans are converted to the corresponding disulfides.
U.S. Pat. No. 5,597,476 discloses a two step process in which naphtha is fed to a first distillation column reactor which acts as a depentanizer or dehexanizer with the lighter material containing most of the olefins and mercaptans being boiled up into a first distillation reaction zone where the mercaptans are reacted with diolefins to form sulfides which are removed in the bottoms along with any higher boiling sulfur compounds. The bottoms are subjected to hydrodesulfurization in a second distillation column reactor where the sulfur compounds are converted to H2S and removed.
In a catalytic distillation column reactor, the reaction temperature is limited by the boiling point of the material in the catalyst bed which is a function of the boiling range of the naphtha and the pressure in the reactor, which may require using higher pressures than desired to obtain the necessary reaction temperatures of desulfurizing light naphtha.
The present process is particularly useful for desulfurizing light naphtha. The central elements of the process comprise a catalytic distillation hydrodesulfurization step in combination with a distillation step and a straight pass hydrogenation step. In the catalytic distillation hydrodesulfurization step mercaptans in the feed are contacted with hydrogen in the presence of hydrodesulfurization catalyst preferably at pressure of  greater than 250 psig and fractionated into a first overheads and a first bottoms. In the distillation step the first bottoms are fractionated into a second overheads and a second bottoms. In the straight pass hydrogenation step the second bottoms is contacted with hydrogen in the presence of a hydrodesulfurization catalyst at pressure of  greater than 250 and temperature  greater than 400xc2x0 F. to further reduce the sulfur content. The light naphtha feed is composed primarily of C4-C10 hydrocarbons (typically C5-350xc2x0 F.) which contain mercaptans and diolefins in amounts generally less than 1000 ppm before processing. The sulfur compounds are treated in the straight pass hydrogenation to extinction ( less than 50 ppm).
Briefly the present process comprising:
a. feeding a naphtha stream containing diolefins and organic sulfur compounds comprising mercaptans to a reaction distillation zone;
b. concurrently in said reaction distillation zone:
i. contacting said naphtha feed with hydrogen in the presence of a hydrodesulfurization catalyst, preferably at a pressure of  greater than 250 psig, to produce a reaction mixture containing H2S and
ii. fractionating said reaction mixture into a first overheads containing H2S and a naphtha fraction of less than C8 and a first bottoms of a C6+ naphtha fraction and organic sulfur compounds boiling in the range of the C6+ naphtha fraction;
c. fractionating the first bottoms into a second overheads containing a C6-C7 naphtha fraction and a second bottoms containing a C7+ naphtha fraction and sulfur compounds boiling in the range of the C7+ naphtha fraction; and
d. contacting the second bottoms with hydrogen in the presence of a hydrodesulfurization catalyst in a hydrodesulfurization zone, preferably at a pressure of  greater than 250 and a temperature  greater than 400xc2x0 F., to further reduce the sulfur content.
In other embodiments the naphtha, without prior treatment to remove diolefins and mercaptans, is fed directly to the catalytic distillation hydrodesulfurization step. In these embodiments the overheads are a wider cut, e.g., C5-C7, and include diolefins and mercaptans which are reacted in a catalytic distillation thioetherification (CDTE) step. The bottoms from the CDTE step containing sulfides are returned to catalytic distillation hydrodesulfurization step and the C5-C6 product is recovered as a side draw from the CDTE step, the overheads being primarily H2S and H2.
In other embodiments the naphtha, without prior treatment to remove diolefins and mercaptans, is fed directly to the catalytic distillation hydrodesulfurization step. In these embodiments the overheads are a wider cut, e.g., C5-C7, and include diolefins and mercaptans which are reacted in a catalytic distillation thioetherification (CDTE) step. The bottoms from the CDTE step containing sulfides are returned to catalytic distillation hydrodesulfurization step and the C5-C6 product is recovered as a side draw from the CDTE step, the overheads being primarily H2S and H2.
As used herein the term xe2x80x9cdistillation column reactorxe2x80x9d means a distillation column which also contains catalyst such that reaction and distillation are going on concurrently in the column (in a reaction distillation zone). In a preferred embodiment the catalyst is prepared as a distillation structure and serves as both the catalyst and distillation structure.